Suspension for trailer with wide loading range

[EsoEng]

I'm designing a small trailer. Its unladen weight will be 50 kg, and fully laden weight will be 250 kg. I need a shock absorber solution to adequately suit both extreme loading scenarios. I have had difficulty identify one. How would a designer within industry solve this problem, please?

I have not been able to identify a single product that's suitable. A suspension company I spoke with advised a custom solution possibly involving two springs in serial around a single damper unit (coil-over). However, I think this solution would not allow sufficient travel when the trailer is fully laden due to excess sag. My thinking is that I could use two coil-overs per wheel: one connected for when the trailer is empty (5- kg), and two connected for when the trailer is full (250 kg). The shock absorber used when fully laden would be disconnected when the trailer is used empty. This is not ideal due to needing two shocks per wheel instead of one, and the increased weight and cost this would bring.

My other option is to use air shocks (one air shock unit per wheel; no other coil overs, springs, or dampers). So, my main question (if a better option is not offered/found) concerns off-the-shelf air shocks (such as those made by Monroe): will I be able to maintain the same ground height using air shocks when the trailer is both empty and fully laden; and, will the suspension provided by the air shocks be satisfactory for both loading extremes? Can I simply pump more air into the air shocks for a fully laden trailer to provide for the extra weight? I have no experience with air shocks, so please entertain my basic questioning!

Thanks.

 

[BrianPetersen]

That is a very small trailer indeed.

Normal trailer suspensions are very stiff, so stiff that dampers aren't used, merely very stiff leaf springs, and they are simply allowed to bounce around when lightly loaded. Any particular reason you don't want to just let it bounce? If the wheels are flying after going over a bump, so what?

How are you guiding the wheels up and down? Trailing arms (independent), or a beam axle guided by some means? Most small trailers use leaf springs because they do both the axle-guiding and the springing in one. If you want to use coil spring and dampers then the axle needs to be guided by a linkage, and that's going to be complicated.

How much vertical wheel travel are you planning for? Most trailers have very high spring rates with the consequence (both a bug, and a feature) of the ride height being almost the same laden as unladen.

Inspiration

http://www.cerka.ca

The torsion-flex axles are a small square tube inside a bigger square tube, with rubber taking up the triangular gaps between them. The geometry is independent trailing-arm. The rubber does the springing and its internal hysteresis does the damping. But, your situation is smaller than the smallest that they allow for. It may provide design inspiration, though.

 

[EsoEng]

Thanks for your reply.

I'm aware of the torsional suspension units you described, and even own a pair. Yes, they are very stiff, so I changed my mind in favour of independent trailing/swing arm suspension with standard shock absorber units.

The trailer is small because it will be pulled by a motorcycle. This is also one reason why it needs to be more stable.

I'm hoping a straight swing arm without linkages will suffice. Each arm, one per wheel, will have one pivot, and connected to the shock absorber close the the wheel axel, similar to old Harleys and Triumphs with their twin rear shocks.

Vertical wheel travel will be a maximum of 90 mm, so very small. The trailer will be used off-road for camping, but mostly on-road.

What are your thoughts on the air-shock solution, please?

 

[BrianPetersen]

Aye, motorcycle trailer makes sense.

Canned solution ...

https://needatrailerpart.com/produc...ug-motorcycle-trailer-camper-short-trike-half

As for "air shocks", I'm not aware of any that are designed to completely substitute for the springs. They're intended to substitute the dampers with the possibility of a "little" extra ride height due to being able to be charged up with air. The damping circuit is designed for the full weight of the car that they're meant to be installed in, and the springing just for that little bit of extra in parallel with a normal spring (in the automotive installation). For what I think you're trying to do, the consequence is probably that the damping circuit is going to be waaaaayyyy too stiff.

Or maybe you're thinking of something like this:

https://www.tdotperformance.ca/seri...NYUYthWZwBwlXnrNXs9Sh-bj7rxF6ctBoC31kQAvD_BwE

I suspect you'll have the same problem - a damping circuit that's meant for supporting the springing of a car. Too stiff for what you're trying to do.

I also don't think there's a way around the damping being at least somewhat wrong ... your laden mass is a factor of 5 more than your unladen mass, and there's just no way to design a single hydraulic damping circuit that's not going to be either too stiff when unladen or too soft when laden (or some of each).

Personally I'd be tempted to go with the bought torsion-axles and call it good enough. At least those don't require you to provide space for shock/spring towers cutting into cargo volume.

 

[cibachrome]

Leaf springs have load dependent friction with multiple leaf stacks.

 

[Tmoose]

Holsclaw

https://www.twtex.com/forums/attachments/1586703473622-png.259237/

Coil springs.
Hydraulic tubular shocks.
Plenty of capacity

I can attest that the tales of legendary excellent towing characteristics are true.

 

[EsoEng]

@BrianPetersen, I'm considering this air-shock:

https://www.amazon.co.uk/Monroe-MA785-Max-Air-Adjust-Absorber/dp/B000C55SVE/

It is currently used in Bushtec motorcycle trailers, and I believe in some motorcycles replacing original coil-overs. I've heard of no issues concering damping being too hard. I am still to allay my lack of confidence concerning controlling ground height while ensuring good suspension for trailer loading extremes.

Thank you for your links, and further reply.


cibachrome, I'm not keen on leaf springs. They would likely be too heavy, and how can they cope with the large weight difference? Thanks.


Tmoose, thanks. The trailer in the image you linked to has a single axel. Mine will have independent suspension. You describe the trailer as having "plenty of capacity." Do you mean it can carry large loads, or that the suspension is suited for low weight and high weight?

 

[BrianPetersen]

There's no technical information in that link on what that Monroe shock is actually meant for. The photos show it being used as a damper with the air acting only for supplementary load-carrying. It's quite possible that in your light application, it could carry the entire load of the trailer. It's also quite possible that the damping is "good enough" even if it isn't "perfect".

It's going to extend (raising the trailer) when you unload the trailer before you get around to letting out some air pressure. It's going to sack down against the bump stops when you load up the trailer before you get around to pumping it up (or if you are caught out somewhere with no access to compressed air). Air suspension is gonna do that.

The high variability in ride height is why the usual approach with trailers is to use really high spring rates ...

I think the rubber torsion springs are designed to have a progressive spring rate, so that when lightly loaded, they're riding higher but in a softer part of the spring rate, then they get stiffer as the load increases (and they ride lower) so that they don't move around excessively.

 

[LionelHutz]

The rubber torsion springs are the worse bouncing trailers on the road. They'd need shocks for sure, and I'm doubtful they'd work better than a solid axle and letting the wheels be the only suspension for the trailer being described.

 

[GregLocock]

RTS are pretty much linear rate. There is a good reason why real trailers use leaf springs, they give a rising rate with load. That still leaves you with the damping problem, but so long as there is /some/ damping it all sorts itself out eventually. An alternative would be to use position sensitive dampers, Fox and others make them, or go the whole hog and use MRDs. That's a bit of a research project. Or use manually adjustable shocks and just set them soft for unladen and hard for laden.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MadisonCameron]

Hello. Install the same suspension as on old cars and then it won’t matter to your trailer whether it has a load of 5 kg or 250 kg. The suspension I'm talking about is something like a spring - hardened metal plates will create the necessary springiness for the trailer and will hold more loads compared to a spring suspension. And you don't have to install shock absorbers. The only negative is that the handling of the trailer will deteriorate significantly.

 

[BrianPetersen]

"The same suspension as on old cars" - "hardened metal plates"?

Would you be talking about "leaf springs", perhaps?

That, with a dead-beam axle, is the most common, and cheapest, and simplest, and crappiest, trailer suspension design.

From earlier posts, I'm thinking the original poster is wanting to minimise that "the handling of the trailer will deteriorate significantly", and with the trailer to be towed behind a motorcycle, I can understand why. And a desire to lower the center of gravity was also mentioned ... and leaf spring with dead-beam axle of the usual design is not good on that count, either.

I still say the rubber torsion spring setup is a good choice; perhaps make an adapter to each trailing arm so that you can connect a regular hydraulic shock absorber (shortest ones you can find) to each side. It will not be possible to have the "ideal" damping curve throughout the trailer's loading range, but this way you can arrange for it to have at least "some" damping, which is better than what most trailers have.

 

[XR250]

What about using motorcycle shocks? They are a spring and damper combined.